Depending on your deck material...
I used marine grade plywood as a top to my drop bag. Cut holes for straps to secure it to frame. I varithaned it with Spar Urethane
. Used 4 to 5 coats. Then added floor flanges
to the top so it would double as a table. (cut 1 in black pipe to desired height and have ends threaded by a metal shop) Then used cheap closed cell foam from an old camping back pad
(found at any camping store for $10... use the blue or yellow avoid the grey or black.. to hot) To glue this down, I used 3M heavy duty spray adhesive (Ace Hardware)
. You spray a light coat on the wood and on the foam and then lay them together. The used a razor knife to trim the edges. It makes for a comfortable seat and kithcken cook table.
Then go buy a paco pad anyway. You will not regret it.